Field
Embodiments of the present disclosure generally relate to layered mixed-metal oxides, and specifically relate to mixed-metal oxide catalysts with high temperature stability.
Technical Background
The synthesis of supported metal or metal-oxide catalysts is of great industrial importance in heterogeneous catalysis. High activity, high selectivity, and long catalyst life are desirable characteristics of any industrial catalyst. Among the metal/metal oxide supported catalysts, the Cu/ZnO/Al2O3 system and metal/metal oxide (Pt, Pd, Rh and Au) systems supported on various supports (alumina, silica, and carbon) have great industrial importance. Generally these catalytic systems are prepared by methods such as sol-gel, deposition-precipitation, deposition-reduction, and impregnation methods. These synthetic methods are challenged with problems such as uneven distribution, deposition of active metal species onto the support leading to the agglomeration of particles and sintering of active species at higher temperature, as well as during recycling.
Mixed-metal oxide materials can be obtained by thermally decomposing layered double hydroxides (LDH) materials. LDHs are also known as anionic clays and are inverse charge analogues of the widely used aluminosilicate cationic clays in structure and properties. LDHs undergo thermal decomposition in three main steps: (a) from room temperature to 100° C., adsorbed/physisorbed water is removed; (b) from 100° C. to 220° C., intercalated water is removed; and (c) from 220° C. to 400° C., intercalated anions are removed and mineral layers are dehydroxylated, leading to the formation of an amorphous mixed-metal oxide residue. Mixed-metal oxide materials formed from heating an LDH to the temperature range of 220 to 400° C. are typically both amorphous and composed of a single-metal oxide phase (MIIO) and a spinel phase (MIIM2IIIO4). The anion that was present in the parent LDH material is generally no longer present in the mixed-metal oxide material or is present to such a small extent that it does not noticeably affect the properties of the mixed-metal oxide material. Unfortunately, further heating of an LDH to greater than 800° C. may result in the formation of a thermodynamically stable and irreversible spinel phase accompanied by phase segregation and sintering.